Potyviruses constitute the largest known group of plant viruses, infecting a wide variety of plants, inter alia, potato, tomato, tobacco and pepper. Potyviruses thus inflict heavy economic damage Hollings, M. & Brunt, A. A. (1981) in Handbook of Plant Virus Infections: Comparative Diagnosis, ed. Kurstak, E. (Elsevier/North Holland), pp. 731-807; Dougherty, W. G. & Carrington, J. C. (1988) Ann. Rev. Phytopathol. 26:123-143!. The organization of the large (9-10 Kb) RNA genome of potyviruses is well understood. One of their major characteristics is that their genome comprises a single, large, open reading frame, encoding a large primary polyprotein. The mature virus-encoded proteins are produced by self-proteolysis, carried out by the products of three of the viral cistrons Verchot, J., et al., (1991) Virology 185:527-535; Hellmann, G. M., et al., (1988) Virology 163:554-562; Carrington, J. C., et al., (1989) EMBO J. 8:365-370; Carrington, J. C., et al., (1988) J. Virol. 62:2313-2320; Garcia, J. A., et al., (1989) virology 170:362-369; Carrington, J. C. & Herndon, K. L. (1992) Virology 187:308-315!. Several non-structural viral proteins tend to accumulate in infected tissues, some in the form of inclusion bodies Dougherty & Carrington, 1988! Matthews, R. E. F. (1991). Plant Virology (3.sup.rd edition), Academic Press, San Diego!. The nuclear inclusion body comprises aggregates of two virus-encoded proteins: NIa and NIb. NIa is the major potyvirus protease, cleaving at all proteolytic sites except the first two in the N-terminal region. Dougherty and Carrington, 1988; Hellmann et al., 1988; Garcia et al., 1989!. NIb is thought to be associated with replication, due to some sequence homology with the polio replicase Allison, R., et al., (1986) Virology 154:9-20; Domier, L. L., et al., (1987) Virology 158:20-27; Robaglia, C., et al., (1989) J. Gen. Virol. 70:935-947!.
Plants transformed with viral structural gene sequences have been shown to acquire resistance to infection by the same, or related viruses reviewed in Beachy, R. N., et al., (1990) Annual Rev. Phytopatol. 28:451-474!. Resistance has also been recently reported for the first time in tobacco transformed with a nonstructural viral sequence, the putative tobacco mosaic virus replicase Golemboski, et al., (1990) Proc. Natl. Acad. Sci. USA 87:6311-6315!.
Potyviruses express a fair amount of nonstructural viral proteins that aggregate into stable forms and thus offer a unique opportunity to study the expression and function of these proteins. In the course of studying the factors controlling NIa expression and function, the inventors transformed plants with the NIa cistron. Surprisingly, these transformed plants exhibited a high degree of resistance to potyvirus infection.